Microscopic appearance of Bradyrhizobium ORS278. The bacteria in the form of a club, have a tendency to agglutinate to form rosettes (Photo IRD).

    Bacteria of the genus Bradyrhizobium are, like the rhizobia, capable of inducing the formation of special organs called nodules on the roots of plants of the legume family, in which they fix atmospheric nitrogen in symbiosis with the plant. These symbioses play a key role in the biologic nitrogen cycle, in agriculture, and in the renewal of degraded soil. Research on these organisms is important for ecology, biology and economics.

Five species of Bradyrhizobium have been described to date, of which B. japonicum, because of its capacity to form nodules on soja, is one of the bacteria which has been exploited most. It genome has been completely sequenced by the Kazusa DNA Research Institute.

The photosynthetic Bradyrhizobia form a separate phylum and may constitute a new species of Bradyrhizobium.

Stem nodule of Aeschynomene sensitiva (Photo IRD)
Loss of photosynthetic function during symbiosis between Bradyrhizobium ORS278 and Aeschynomene sensitiva (Photo IRD)
Pigmentation of Bradyrhizobium ORS278 colonies. The red-orange color is due to the production of 2 pigments: spirilloxanthine (a photosynthetic carotenoid) and canthaxanthine (the principal pigment, which represents up to 80% of the carotenoids produced)(Photo IRD).

They are phylogenetically close to Rhodopseudomonas palustris, a purple bacterium whose genome has been sequenced by the DOE Joint Genome Institute. This bacterium is known by microbiologists for its adaptability to extremely variable environmental conditions and its capacity to degrade several xenobiotic compounds. The photosynthetic Bradyrhizobia, which occupy an intermediate phylogenetic position between B. japonicum and R. palustris, are characterized by several remarkable properties:

1. The capacity to form nodules on the stems of some tropical aquatic legumes of the genus Aeschynomene. These shoot nodules may be profuse, and plants with this type of nodules have an exceptional capacity for nitrogen fixation. These legumes are utilized in some regions of Asia and Africa as green fertilizer, especially in the cultivation of rice. The ORS278 strain was isolated in Senegal from a stem nodule of Aeschynomene sensitiva in 1991.
   
   
2. The ability to photosynthesize, which is a unique property in these rhizobia. It has been shown that this photosynthetic activity plays a key role in symbiosis: during the first stages of the intraction with the plant by facilitating survival and infectivity of the bacterium, and during symbiosis with the shoot by furnishing energy which can be used for biological nitrogen fixation. The regulatory mechanisms for the formation of this photosystem are very different from those described in purple bacteria. In fact, a bacteriophytochrome is involved, which, as a function of the ambient light, triggers or fails to trigger the expression of photosynthetic genes. This original mechanism of regulation seems especially well adapted for promoting a specific photosynthetic activity during symbiosis with the stem.
   
   
3. The capacity to colonize the root system of rice. These photosynthetic Bradyrhizobia are found to be naturally associated with a wild rice species, Oryza breviligulata which co-inhabits some tropical marshes of Africa with Aeschynomenes. This association is very beneficial for the growth of the plant and the production of seeds; plots inoculated with the ORS278 strain achieve production yields which are 20% higher.
   
   
4. The capacity of some strains, and especially the ORS278 strain, to produce canthaxanthine. This carotenoid of biotechnological interest is used in the agro-alimentary, pharmaceutic and cosmetologic industries for its coloring and photo-protective properties.
   
   

This sequencing project thus has as its principal objectives: to enlarge the knowledge available in genomics of temperate rhizobia to the tropical milieu; to contribute to the comprehension of the mechanisms of plant/bacterial interactions (Bradyrhizobium/rice and Bradyrhizobium/legume); to elucidate the mechanisms of adaptation of the bacterium to different light environments and to understand the evolution of symbiotic and photosynthetic systems in rhizobia through comparative genome analysis.

Trial inoculation of a rice paddy with Bradyrhizobium ORS278 (Conakry Guinea - Photo IRD)